In recent years increasing research has been focused on the realization of optoelectronic integrated circuits (OE-ICs) on silicon. Possible applications would be chip-to-chip interconnects, parallel processing and the integration of photonics on silicon chips. While the first two applications require basically a light source and a detector on silicon, operating above 77 K, the last application requires the operation of the light source at a certain wavelength, i.e., about 1.5 .mu.m, which falls in the absorption minimum of optical fibers.
In 1983 Ennen et al. [Appl. Phys. Lett. 43, 943 (1983)] pointed out the potential of rare-earth ions in semiconductor materials for the development of light-emitting diodes and lasers. One of the most promising candidates for the preparation of these devices is erbium doping of silicon. The 1.54 .mu.m luminescence of erbium is below the band gap of silicon, thus allowing the construction of optical wave guides within the silicon. This property presents exciting possibilities for creating optical devices in silicon and for integrating electrical and optical devices in circuits fabricated in silicon. The mature manufacturing technology of silicon can be extended into optical communications by this path as the limitation of the silicon indirect band gap is overcome. This wavelength is also becoming extremely important in optical communication because it corresponds to a transmission maximum in optical fibers and is also the output wavelength of IR-pumped Er-doped silica optical amplifiers.
The 1.54 .mu.m luminescence of erbium is the result of an internal 4f transition. The 5s and 5p shells shield the 4f orbitals of the Er.sup.3+ from first-order host lattice effects, and, thus, luminescence is fairly independent of the host materials. The optical transitions occur between the spin-orbit levels, .sup.4 I.sub.13/2 .fwdarw..sup.4 I.sub.15/2, of Er.sup.3+ (4f.sup.11). Since the influence of the crystal field of the host lattice is weak, erbium as an impurity in silicon is expected to show luminescence at room temperature.
Within the past decade, the photo- and electroluminescence, electrical characteristics, and structural properties of Er-doped silicon have been studied. However, prior to the present invention, all Er-doped silicon layers had to be prepared by ion implantation of bulk silicon or by low energy ion implantation of MBE grown silicon. After implantation, samples were annealed to both remove ion damage and to "activate" the implanted erbium. (Activate in the sense of possibly forming an Er-impurity complex which acts as the optical center in these materials.) The best results were obtained at annealing temperatures of 900.degree. C. Unfortunately, erbium possesses a solubility limit in Si of about 1.3.times.10.sup.18 atom/cm.sup.3 at 900.degree. C., and annealing results in the formation of platelets of ErSi.sub.2 which precipitate out within the silicon phase if the concentration of Er is higher than 1.3.times.10.sup.18.
Since higher levels of incorporation of rare earth into epitaxial silicon layers would provide more efficient and powerful devices, there is a need for a process which would produce levels of incorporation above the present limit of solubility at 900.degree. C.